If the driver of a vehicle forgets to engage or fails to properly engage the parking brake of the vehicle prior to leaving it when parked on a slope, the vehicle may under its own weight start rolling away. Such accidental situations may cause serious personal injuries and property damage, especially when it comes to heavy motor vehicles such as lorries, towing vehicles or buses. Motor vehicles, such as cars, lorries, towing vehicles and buses, are often provided with a conventional warning function which is activated so as to generate a warning signal when the driver turns off the ignition of the vehicle without having engaged the parking brake, so as to thereby alert the vehicle driver of an inadvertent failure to properly engage the parking brake. On many occasions, however, it is desired to leave the engine running while leaving the vehicle cabin for a short time, e.g. in order to clean the windshield, check a tire, make a delivery etc. On these occasions, said conventional warning function will not be activated due to the fact that the ignition is on when the driver leaves the vehicle cabin, irrespective of whether or not the parking brake is properly engaged. Thus, there is a need of a warning system that may alert the driver that the parking brake is not properly engaged when he/she is about to leave his parked vehicle with the engine still running.
For a parking brake warning system to be efficient, it must generate a rather powerful warning signal so as to insure that the driver before leaving the vehicle really perceives the signal and is alerted of the failure to properly engage the parking brake. However, the warning system should not generate warning signals undesirably, e.g. in situations where the driver, after having stopped the vehicle, intends to remain seated in the vehicle with the parking brake disengaged in order to reverse the vehicle or operate the vehicle in another manner. Such undesired warning signals will annoy the driver and could, if frequently occurring, make the driver less attentive to the warning signal in a situation when he/she actually has omitted to engage the parking brake or even make the driver disable the entire warning system.
A warning system for a motor vehicle provided with a parking brake is previously known from U.S. Pat. No. 3,651,457 A. This known warning system will properly generate a warning signal when the driver opens the driver access door of the vehicle cabin in order to leave the vehicle without having engaged the parking brake. However, this warning system will also generate a warning signal in a situation when the driver intends to remain seated in the vehicle with the driver access door open and the parking brake disengaged, e.g. in order to reverse the vehicle while looking out through the open door. Thus, this known warning system suffers from the above-indicated drawback of undesired warning signals.
A warning system that hypothetically could alleviate the above-indicated problem of undesired warning signals is previously known from U.S. Pat. No. 4,967,182 A. This warning system uses a pressure sensor to sense the pressure on the driver's seat and to close an electric switch whenever the seat is unoccupied. A warning signal is generated if the seat is vacated without the parking brake of the vehicle having been engaged. However, sensors for detecting whether or not the driver's seat is occupied are normally not included as standard components in motor vehicles, which will make it rather costly to implement this known warning system. Furthermore, existing sensors of this type are in general not very reliable, which may result in annoying warning signals during driving or in the lack of warning signals when the driver leaves the vehicle without having engaged the parking brake.